This invention relates to a broadbrand electric field controlled switching circuit and in particular, to such circuit having two or more electric field controlled switch elements coupled in series between an input and an output of the circuit.
Electric field controlled switching elements, such as field effect transistors (FET's) are generally utilized in switching applications where a desired high on/off resistance ratio and zero DC offset is required. The major factor which limits the use of FET switch elements in broadband applications is the trade-off required between obtaining low values of feedthrough impedance Rds(on) between the drain and source when the switch is in the on mode and obtaining high values of feedthrough impedance in the off mode. The major contributors to the value of off mode impedance are parasitic capacitances primarily between the drain and gate (Cdg) and between the source and gate (Csg) which affect signal feedthrough and bypass in the off mode. There also is a small parasitic capacitance between the drain and source (Cds).
Current art favors the use of FET's with low parasitic capacitances and high Rds(on) (above 50 ohms). These FET's can be used to pass a broadband signal when used as a series switch between a low impedance signal source and a low impedance load (less than 100 ohms). However, the signal losses in Rds(on) compromise switch performance as the series Rds(on) results in the loss of a large portion of the signal. Also this loss varies with Rds(on) which value in turn varies over a wide range with commercial FET's. Because of this signal loss, additional amplification circuitry and an individual gain calibration control may be required.
As the frequency of the transmitted signal increases, feedthrough impedance in off mode decreases, thus making a single FET incapable of providing sufficient isolation between the input and output. Multiple FET's can then be arranged in networks which generally are referred to by names suggestive of the shape of the interconnecting active elements. These networks conventionally include an "L" section where a parallel or shunt-to-ground FET is added before or after the basic series off/on FET and controlled in opposite on/off conductance manner. Another conventional configuration is to have a "T" network which includes two series FET switches and a shunt FET switch connected between the two interconnected switches and ground. The shunt FET conventionally provides a low impedance bypass when the series switches are in an off mode. However, at higher frequencies, the parasitic capacitances in this shunt circuit provide increasingly lower impedances when the series FET's are in an on mode, thereby increasing the signal loss in the circuit. While at low frequencies these networks offer satisfactory performance, in broadband applications extending into the VHF region, performance becomes marginal due to previously described signal losses and gain variation inherent to all FET's in the circuit.
In multichannel applications utilizing two or more parallel signal channels, for example in a signal multiplexer or demultiplexer, where each channel has conventional series FET switches and FET shunt elements, two control voltages are required for each channel. That is, when the series switches are on, the shunt switch or switches must be off, and vice versa. It is therefore desirable to provide switch control using a single control voltage polarity per channel.